In wireless mobile code division multiple access (CDMA) communication systems, capacity and coverage are inter-related in a manner dictated by some key principles--the radio links are designed to operate at a minimum level consistent with voice quality; coverage area is dictated by area where the mobile station has sufficient power to achieve the target bit energy-to-noise ratio at the cell site (also known as the base station and referred to as BS) receiver. These considerations lead to fundamental design tradeoffs in CDMA systems--design coverage increases with lower capacity; coverage and or capacity increases with reduced voice quality.
CDMA is a modulation and multiple access scheme that is based on spread spectrum communications. CDMA signals are generated by modulating the data with pseudo-noise codes (for the particular case of direct sequence CDMA systems, these PN codes are a sequence of chips taking on values +1 or -1) which are signature codes assigned to individual users (herein referred to as mobile stations or MS). In a wireless mobile communication system, an MS communicates directly with the fixed entity, BS. The BS receiver identifies the individual MS by de-spreading the pseudo-noise signature codes and information data is extracted by demodulation.
CDMA communication systems are subject to one basic limitation that effects operation and design: all users are inherently interfering signals to all other users. This effects the CDMA system architecture and determines the limit (maximum theoretical user number) on how many interfering users can be present before system operation degrades to unacceptable levels. Additionally, maximum user capacity calculations inherently presume that all conditions are ideal. In a real world application of most systems this is not true, and CDMA systems are no different. With typical CDMA systems, even using perfectly orthogonal spreading codes, issues related to practical deployment become the main driver in the system (and therefore a network of systems) capacity calculations and functional operation. Each cell is surrounded by other cells that are transmitting similar (interfering) signals. Further, all users are not an equal distance from the base station causing each user's signal to be received with unequal strength compared to all other users (although user power control is attempted with modest success). In addition, the RF propagation environment provides multiple opportunities for signals to be reflected thereby causing unwanted signal strength variations at both the base station and the mobile station (Rayleigh fading, another effect attempted to be controlled by implementation of user power control). Together, these effects contribute to a reduction in the total system capacity, compared to theoretical maximums, along with variable system service coverage areas that change based on the number of users of the system. Additionally, implementation inaccuracies of the various algorithms in hardware and software typically further restrict the total number of users to a practical maximum limit.
Since each user is an interfering signal to all of the other users, the chosen system implementation (spreading code and bandwidth, data transmission rate, power control implementation, base station layout, and "soft" handoff algorithm and implementation) determines the total number of users which, as an undesired by-product, also determines the coverage area. Users with higher power than others present stronger interfering signals than weak power users, and therefore limit the overall number of users on the system by virtue of their signal strength. Once a system architecture has been chosen, CDMA system capacity is determined by the total interference power in the system; this power level can be generated by several high power users or by many more users of equally low power. The effect can be described as follows. When there are few users on the system, cells are relatively far apart, the coverage area of a typical cell is large, there are few interfering signals competing at the base station for network access, and weak signals (at the minimum designed receive signal level) are recognized and accepted. As more phone calls are accepted into the network the success of the system's ability to control each user's power becomes much more important. Those users closest to the base station would have a decided advantage over those far away, if no power control were implemented, since stronger users have a decided advantage in making phone calls (up to the maximum practical user limit). Typical wireless mobile phone CDMA systems address this situation by attempting to control the power transmitted by each user such that those both near to and far from the base station have the same power level at the receiver, and therefore an equal chance to acquire network resources (a "channel"). The measures taken to accomplish this are not completely successful, which accounts for some of the differences in capacity between expected theoretical and practical user limits of the typical CDMA system. The net effect of imperfect power control and real-world fading environments combined with a maximum practical user limit on each served area is that users further from the base station receive access only if the total user count in each served area is less than the maximum practical limit associated with that CDMA system. Given the inaccuracies of the power control implementation, even close users can be dropped from service if users closer to the base station request service and receive a channel to make a phone call. This effect has been referred to as the "breathing cell" phenomenon, meaning that the coverage area of a CDMA cell depends on how many users are accessing the system, their power, and their location at any time. As a result, CDMA service providers have trouble in determining where to place base stations in a network. It is difficult, if not impossible, to determine exact coverage areas based on typical RF propagation effects since the number of users, their location, and success of power control of the system determines the coverage pattern.
This "cell breathing" issue has been typically addressed in a brute force manner. The service provider essentially doubles the number of base stations covering a certain geographic area, compared to typical TDMA network layouts, such that most of the area is covered by at least two base stations, and sometimes three or more. This technique attempts to ensure that continued coverage is provided for users who have initiated phone calls and are being served by the network even when other users closer to the base station make phone calls into the network. Since the signal from the mobile in question is strong at several base stations, the phone call may be maintained if it is transferred from one base station to the other. Thus, the user in danger of being dropped has the call transferred by the network to another base station that has less than the maximum number of users so that the call is maintained. When the number of users of both base stations has reached the practical user limit, any new users vying for service are denied access. As a practical matter, this limit will eventually be reached in any system given more customers in a service area than the design limits of the CDMA.
In any mobile cellular system there exist two fundamental problems that a system designer has to deal with.
Multi-path fading of the radio link. This is a phenomenon where radio frequency (RF) signals when transmitted from either the MS or BS traverse multiple paths due to reflections off different objects in the environment before they arrive at the receiving antenna. These multiple reflected paths or multi-path components combine, either constructively or destructively, to produce fades in signal strength. PA1 Multiple access interference or MAI. In CDMA systems, all MS transmit on the same frequency when communicating with the BS and therefore, as mentioned, each MS is a source of interference to every other MS. The level of MAI, to a first degree of approximation, is directly proportional to the number of MS signals received at the serving BS.
CDMA systems exploit the wideband characteristic of the spread spectrum waveforms to resolve the multi-path components and thus, provide the receiver with several independently fading signal paths. This path diversity is exploited by the use of a RAKE receiver to combine the different multi-path components. The same wideband nature of the CDMA signals is used to mitigate the MAI.
The link from the MS to BS is typically asynchronous and such a system is vulnerable to the near-far problem, that is, the problem of very strong undesired MS signals at the receiver swamping out the effects of a weaker, desired MS's signal. A solution to the near-far problem is the use of power control, which attempts to ensure that all signals from the mobiles within a given cell coverage area arrive at the BS of that cell with equal power. Coverage area or range performance is determined by the serving BS. A critical variable is the ratio of single bit energy-to-noise, EbNo, which is analogous to the signal-to-noise ratio in analog systems. An MS has to adjust power--increase or decrease--very fast to achieve target EbNo at the cell site receiver in an attempt to overcome the effects of multi-path fading. But when the MS is close to the edge of the BS coverage area it may be transmitting at peak power output. As the number of the MS increase, the MS at the cell edge will not be able to increase transmit power enough to achieve the target EbNo. Consequently, the range or coverage is sacrificed, resulting in the BS dropping the MS at the cell edge.